David Cameron’s authority received an embarrassing blow last night as Tory grassroots activists voted to oust a sitting female MP at the end of a bitter reselection battle.

Anne McIntosh, one of only three women to represent the Conservatives in the north of England, lost an acrimonious campaign which saw allegations of misogyny, dirty tricks and cronyism fly between warring groups in the affluent rural constituency of Thirsk and Malton in North Yorkshire.

Ms McIntosh said she would fight to keep her seat. “I do not intend to be thrown aside by a small group. It is for my constituents as a whole to dismiss me if they wish to do so,” she said as left the vote at Conservative HQ in London. Despite her determination to continue, the result of the secret ballot of 560 members means Mr Cameron faces losing a fifth female MP who will not now be standing in 2015.

The chair of the Environment and Rural Affair Select Committee, who has been highly visible in the media during the recent floods, has previously fought off three attempts to unseat her in the Tory rural heartland. She won at the 2010 general election with an 11,000 majority, making her the only female Conservative in the whole of Yorkshire and Humberside.

Among those tipped to replace her is Edward Legard, an Old Etonian barrister and part-time judge. Critics within the local party claimed Ms McIntosh had demonstrated a “consistent pattern of non-co-operation, non-communication, bad manners and divisive behaviour”.

Her supporters allege she was the victim of an old boys’ network that refused to tolerate a woman as its MP.

One senior local party figure reportedly described the MP – a former advocate who speaks five languages – as a “silly girl”, and suggested the constituency needed someone like Boris Johnson or Nigel Farage to represent it.

The ill-tempered contest has been rumbling on for a year since Ms McIntosh refused to accept the decision of the association’s executive committee to deselect her last January. The saga has been described as the “Tory Falkirk”, after the Scottish seat where Labour faced allegations of trade union vote-rigging.

A Conservative inquiry ordered by central office found the make-up of the committee had broken party rules – although opponents, including the local party chairman, said that the reconfiguration of the governing body had only been deemed unconstitutional when it voted to drop Ms McIntosh.

The MP is often seen sitting behind Mr Cameron during Prime Minister’s Questions. He wrote to her describing her as “one of our most assiduous MPs” and praising her appearances on BBC Radio 4’s Farming Today.

But Labour criticised the Prime Minister for not doing enough to save her. The shadow Cabinet Office minister Jon Ashworth said: “Already one in 10 of the Conservative women MPs elected in 2010 have announced that they are standing down at the next election. David Cameron has done nothing to stand up for one of his most senior women MPs.”

The former environment minister Tim Yeo is also facing deselection in his South Suffolk constituency. The result of a secret ballot of members is due next week. Last September grassroots activists failed to unseat the former prisons minister Crispin Blunt.